1. Field of the Invention
The present invention is directed to a measuring device for measuring the intermediate circuit voltage of a gradient amplifier of a magnetic resonance imaging (MRI) apparatus.
2. Description of the Prior Art
In a typical magnetic resonance imaging apparatus, a number of gradient coils (usually three gradient coils for the x, y and z axes, respectively) are employed to spatially encode the magnetic resonance signals arising from an examination subject after excitation of nuclear spins in the subject. Each gradient coil is operated by a gradient amplifier, and each gradient amplifier typically includes a number of intermediate amplifier circuits or stages. Ideally, it would be desirable to measure or monitor the intermediate circuit voltage of each of these intermediate circuits, however, due to their large number this is impractical for reasons of cost and complexity. An additional problem in measuring the intermediate circuit voltage under such circumstances is that this voltage is usually a floating voltage, i.e., it is not always referenced to a reference voltage or ground.
For these reasons, generally only monitoring of one voltage limiting value or two voltage limiting values takes place whereby a threshold detector is driven by a power Zener diode. If more information is needed, measuring circuits in the intermediate circuit connected parallel to the discharging resistor of the intermediate circuit capacitor are utilized. Such measuring circuits, however, are not only considerably more expensive and require considerably more outlay in terms of circuitry, but also still require an auxiliary power supply.